1. Technical Field
The invention relates to a method for allocating transmission conditions to transmit nodes in a wireless network. In particular, the invention relates to allocating transmission conditions in a network having a receive node and a plurality of transmit nodes, in which the receive node is subject to interference from at least some of the transmit nodes when they transmit data signals over the network.
2. Description of Related Art
A general definition of an ad hoc network is a collection of mobile nodes, which communicate with each other over a wireless channel. By definition, ad hoc networks have no fixed infrastructure. All network functions have to be coordinated in a distributed manner between the network nodes. Central to the idea of ad hoc networks is the concept of multi-hop, where each node can act as a router and forward packets on behalf of other nodes towards their destination. This is in radical contrast to conventional cellular systems where each mobile device communicates directly to a base station, which controls all transmission and routing functions. An essential feature of ad-hoc networks, which has no parallel in wired networks, is the relationship between power control, call admission control, network topology and routing algorithms.
There are a number of parameters that can be used to control (and improve) the performance of the physical layer of ad hoc wireless networks. For example, modulation, transmit power, spreading code and antenna beams. By controlling these transceiver parameters adaptively and in an intelligent manner, the capacity of the system can be increased significantly. In earlier research works, power control, antenna beamforming and channel assignment or channel allocation were either done separately or in a combination of some of the above-mentioned techniques. Here we consider three synergistic parameters—transmit power, antenna beamforming and channel assignment as they are intuitively the easiest to exploit and perhaps the most studied. The benefits of antenna beamforming include reduced interference due to narrower beamwidth, longer range due to higher signal to noise and interference (SINR) ratio, and improved resistance to jamming. The benefits of power control include reduced interference and lower energy consumption. Channel assignment is mostly involved in frequency planning, channel reuse schemes and network capacity optimisation to limit network interference while maximizing the system capacity. Future techniques in interference mitigation in wireless networks may combine power control, antenna beamforming and channel assignment techniques to enable higher capacity due to increased spatial reuse, channel reuse, lower latency, better connectivity, longer battery lifetime, and better security (robustness to eavesdropping).
In the paper “Performance evaluation of distributed measurement-based dynamic channel assignment in local wireless communications” by M. M.-L. Cheng and J. C.-L. Chuang, IEEE Journal on Selected Areas in Communications, Vol 14, No. 4, pp. 698-710, May 1996, a method of channel allocation in wireless networks is proposed based on the least interference criterion. The idea behind this technique is that selecting the channel with the least interference requires the least transmission power to maintain the SINR threshold. Later Kulkarni and Srivastava (2002) addressed the problem of spatially reused frequency channels in FDMA based ad hoc networks by using adaptive modulation techniques described in “Channel Allocation for OFDMA based Wireless Ad-hoc Networks” by G. Kulkarni, V. Raghunathan, and M. B. Srivastava, SPIE International Conference on Advanced Signal Processing Algorithms, Architectures, and Implementations, Seattle, Wash., July 2002. Power control is then used to maintain the minimum QoS requirement caused by frequency reuse. The main objective of their study is to find an assignment of frequencies (channels) and transmission power levels such that the SINR threshold can be attained for all transmitter and receiver nodes. The authors' channel assignment algorithm is based on rate requirements for each pair of links. The links are first sorted in descending order according to their rate requirements. As such this algorithm is also known as minimum incremental power algorithm. At each step, a link which is not assigned a channel is chosen with the highest rate requirement. It is hoped that by assigning a channel to the link there is a least increase in the total transmission power over the entire network. If the assigned channel is not used by any other link, the increase in the total transmitted power is simply the power that is transmitted by the new link. Otherwise the transmitted power needs to be increased due to additional interference.
Another approach to mitigate co-channel interference effects and increase the network capacity is to avoid strong interferers by dynamically assigning the channels to the users as described by D. J. Goodman, S. A. Grandhi and R. Vijayan, “Distributed dynamic channel assignment schemes”, Proc. IEEE Vehnicular Technology Conference, pp. 532-535, 1993. Of late there has been much research on integrating distributed dynamic channel and power allocation (DCPA) schemes. However the DCPA schemes do not integrate power control and channel assignment as one entity but rather are done separately. In the paper by A. H. M. Rad and V. W. S. Wong titled “Joint optimal channel assignment and congestion control for multi-channel wireless mesh networks”, Proc. of IEEE International Conference on Communications (ICC'06), Istanbul, Turkey, June 2006, a joint optimal channel assignment and congestion control (JOCAC) method is proposed using a decentralized utility maximization problem with constraints arising from interference of neighbouring transmissions. To make efficient use of the available wireless resources their work considers both the non-overlapping and partial-overlapping channels in the algorithms. However this work does not consider the stability conditions that influences the channel selection for each link in depth.
Therefore there is a need for an improved method of optimising the number of transmit or sender nodes that can be permitted to transmit data signals in a wireless network.